Apple Computer Inc.’s Mac OS X software has been hit by a mischievous instant messaging virus — the first ever to target the operating system.The virus, dubbed Leap-A by antivirus company Sophos PLC, apparently spreads using Apple’s iChat IM service, forwarding itself as a file called “latestpics.tgz” to an infected user’s buddy contacts, according to information from U.K.-based Sophos.

Clicking on the file allows the malware to install and disguise itself as a harmless-seeming .jpeg icon.

Leap-A is believed to have originally been posted on a Web site for Apple users, posing as a software update. Although the virus is benign and is not believed to be spreading in large numbers, it still marks a minor landmark for a system that has come to be seen in some quarters as immune to such mundane security issues.